Electronic timepiece utilizing oscillator circuits controlled by quartz crystal vibrators to generate a timebase signal are widely utilized at present. Such timepieces can provide reasonably high timekeeping accuracy, with a timekeeping rate variation of the order of several seconds per month or several tens of seconds per year being generally attained. However it has hitherto not been practicable to provide an electronic timepiece which can be economically manufactured on a mass production basis, which would provide a timekeeping accuracy of the order of one second of rate variation per year, or less, without special measures being adopted such as selection of quartz crystal vibrators having rigidly specified characteristics. This is especially true of electronic watches, which must be of very compact construction and have a minimal level of power consumption. Generally, electronic watches employ an oscillator circuit controlled by an XY-cut quartz crystal vibrator, operating at a relatively low frequency of oscillation, i.e. a quartz crystal vibrator which operates in a mode resembling that of a tuning fork. Electronic clocks, on the other hand, generally employ a timebase oscillator circuit controlled by an AT-cut quartz crystal vibrator, which operates in a transverse vibration mode. Such a timebase oscillator circuit has an intrinsically higher level of power consumption than an XY-cut crystal vibrator controlled timebase oscillator circuit, since it operates at a substantially higher frequency of oscillation and also because of increased losses in the quartz crystal vibrator due to the different manner in which the vibrator is supported. However, each type of timebase oscillator circuit has both advantages and disadvantages in relation to the other type. For example, an oscillator circuit controlled by a XY-cut quartz crystal vibrator, operating for example at 32 KHz, displays a frequency/temperature characteristic having second order curvature, i.e. having a single frequency maximum, and so the variation in frequency of such an oscillator circuit due to temperature variations can be easily compensated, over a wide range of operating temperatures, by compensation means which are well known in the art. On the other hand, such a quartz crystal vibrator has rather poor aging characteristics, i.e. the oscillation frequency will drift gradually, over a long period of time. Such frequency drift is an inherent feature of an XY-cut quartz crystal vibrator. With an oscillator circuit employing an AT-cut quartz crystal vibrator, however, a much lower degree of frequency drift due to aging will occur. This low rate of frequency drift is inherent in such a quartz crystal vibrator, and is due to such factors as the very high frequency of oscillation, typically of the order of several magaherz, etc. But such an oscillator circuit has the disadvantage that the frequency/temperature characteristic displays third order curvature, and because of this it is difficult to compensate the frequency of oscillation of such an oscillator against frequency variations due to operating temperature changes, over a relatively wide range of temperatures.
Generally speaking, if the highest possible degree of timekeeping accuracy is desired, it is preferable to utilize an AT-cut quartz crystal vibrator type of timebase oscillator circuit. This is because, if a good speciman of such a quartz crystal vibrator is selected, then the variation of oscillation frequency with respect to temperature can be reasonably small, and because of this, in conjunction with the very low rate of frequency drift due to aging, a timekeeping rate accuracy of the order of several seconds per year can be obtained. However, such a method is not practicable for economical mass-production of electronic timepieces.
There is therefore a requirement for an electronic timepiece having a timebase oscillator circuit with a high degree of timekeeping accuracy over a wide operating temperature range, such as can be obtained by applying temperature compensation means to an oscillator circuit controlled by an XY-cut quartz crystal vibrator, and which also has a very high stability of timekeeping accuracy with respect to aging of the quartz crystal vibrator, such as can be obtained by a timebase oscillator circuit controlled by an AT-cut quartz crystal vibrator. Such a timebase oscillator circuit would eliminate the disadvantages of each type of quartz crystal vibrator, described above, and combine their advantages, and would enable a timekeeping rate accuracy of within one second per year to be attained. This objective is attained by an electronic timepiece according to the present invention, as described hereinafter. This is achieved by utilizing the output signal from an oscillator circuit controlled by an XY-cut quartz crystal vibrator as the timepiece timebase signal, with suitable temperature compensation being applied to this signal, and by utilizing the output signal from an oscillator circuit controlled by an AT-cut quartz crystal vibrator as a frequency reference, for detecting and compensating any changes in the timebase signal frequency caused by aging of the XY-cut quartz crystal vibrator, with this detection and compensation being only carried out when the operating temperature of the timepiece components is within a specific narrow range, having a width, for example, of the order of 1.degree. C. In this way, temperature-induced variations in the oscillation frequency of the AT-cut quartz crystal vibrator have no effect upon the aging compensation operation, so that a timebase signal source of an electronic timepiece according to the present invention can be economically and practicably implemented.